Network Working Group T. Chown
Request for Comments: 4076 University of Southampton
Category: Informational S. Venaas
UNINETT
A. Vijayabhaskar
Cisco Systems (India) Private Limited
May 2005
Renumbering Requirements for StatelessDynamic Host Configuration Protocol for IPv6 (DHCPv6)
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
IPv6 hosts using Stateless Address Autoconfiguration are able to
configure their IPv6 address and default router settings
automatically. However, further settings are not available. If
these hosts wish to configure their DNS, NTP, or other specific
settings automatically, the stateless variant of the Dynamic Host
Configuration Protocol for IPv6 (DHCPv6) could be used. This
combination of Stateless Address Autoconfiguration and stateless
DHCPv6 could be used quite commonly in IPv6 networks. However, hosts
using this combination currently have no means by which to be
informed of changes in stateless DHCPv6 option settings; e.g., the
addition of a new NTP server address, a change in DNS search paths,
or full site renumbering. This document is presented as a problem
statement from which a solution should be proposed in a subsequent
document.
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RFC 4076 Renumbering for Stateless DHCPv6 May 2005
Table of Contents
1. Introduction ...................................................22. Problem Statement ..............................................33. Renumbering Scenarios ..........................................33.1. Site Renumbering .........................................43.2. Changes to a DHCPv6-assigned Setting .....................44. Renumbering Requirements .......................................45. Considerations in Choosing a Solution ..........................46. Solution Space .................................................57. Summary ........................................................58. Security Considerations ........................................69. Acknowledgements ...............................................610. References .....................................................610.1. Normative References .....................................610.2. Informative References ...................................61. Introduction
IPv6 hosts using Stateless Address Autoconfiguration [2] are able to
configure their IPv6 address and default router settings
automatically. Although Stateless Address Autoconfiguration for IPv6
allows automatic configuration of these settings, it does not provide
a mechanism for additional non IP-address settings to be configured
automatically.
The full version of the Dynamic Host Configuration Protocol for IPv6
(DHCPv6) [3] is designed to provide both stateful address assignment
to IPv6 hosts, as well as additional (non IP-address) configuration
including DNS, NTP, and other specific settings. A full stateful
DHCPv6 server allocates the addresses and maintains the clients'
bindings to keep track of client leases.
If hosts using Stateless Address Autoconfiguration for IPv6 wish to
configure their DNS, NTP, or other specific settings automatically,
the stateless variant [4] of DHCPv6 could be used. This variant is
more lightweight. It does not do address assignment; instead, it
only provides additional configuration parameters, such as DNS
resolver addresses. It does not maintain dynamic state about the
information assigned to clients, and therefore there is no need to
maintain dynamic per-client state on the server.
This combination of Stateless Address Autoconfiguration and stateless
DHCPv6 could be used quite commonly in IPv6 networks.
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RFC 4076 Renumbering for Stateless DHCPv6 May 20052. Problem Statement
A problem, however, lies in the ability, or lack of ability, of
clients using this combination to be informed of (or to deduce)
changes in DHCPv6-assigned settings.
While a DHCPv6 server unicasts Reconfigure messages to individual
clients to trigger them to initiate Information-request/reply
configuration exchanges to update their configuration settings, the
stateless variant of DHCPv6 cannot use the Reconfigure mechanism
because it does not maintain a list of IP addresses (leases) to send
the unicast messages to. Note that in DHCPv6, Reconfigure messages
must be unicast; multicast is not allowed.
Thus, events including the following cannot be handled:
o Full site renumbering
o DNS server change of address
o NTP server change of address
o A change in DNS search paths
It would be highly desirable that a host using the combination of
Stateless Address Autoconfiguration and stateless DHCPv6 could handle
a renumbering or reconfiguration event, whether planned or unplanned
by the network administrator.
Note that the scope of the problem could extend beyond Stateless
DHCPv6, since only IP address options have a lifetime; i.e., there is
no mechanism even in the full DHCPv6 that "expires" old information
or otherwise forces a client to recheck that new/updated information
is available. However, with full DHCPv6, a node may learn of updates
to non-address options when renewing its address lease.
3. Renumbering Scenarios
There are two main scenarios for changes to DHCPv6-assigned settings
that would require the client to initiate an Information-request/
reply exchange to update the configuration.
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RFC 4076 Renumbering for Stateless DHCPv6 May 20053.1. Site Renumbering
One of the fundamental principles of IPv6 is that sites receive their
IPv6 address allocations from an ISP using provider-assigned (PA)
address space. There is currently no provider-independent (PI)
address space in IPv6. Therefore, a site changing its ISP must
renumber its network. Any such site renumbering will require hosts
to reconfigure both their own address and default router settings and
their stateless DHCPv6-assigned settings.
3.2. Changes to a DHCPv6-assigned Setting
An administrator may need to change one or more stateless
DHCPv6-assigned settings; e.g., an NTP server, DNS server, or the DNS
search path. This may be required if a new, additional DNS server is
brought online and is moved to a new network (prefix), or if an
existing server is decommissioned or known to be unavailable.
4. Renumbering Requirements
Ideally, any of the above scenarios should be handled automatically
by the hosts on the network. For this to be realised, a method is
required whereby the hosts are informed that they should request new
stateless DHCPv6-assigned setting information.
The solution to the problem may depend on whether the renumbering or
configuration change is planned or unplanned, from the perspective of
the network administrator. There is already work underway toward
understanding the planned renumbering [5] scenario for IPv6 networks.
However, there is currently no mechanism in stateless DHCPv6 for
handling planned renumbering events.
5. Considerations in Choosing a Solution
A number of considerations could be listed for a desirable solution:
o The solution should support planned renumbering; it is desirable
that it also supports unplanned renumbering.
o Security is important. No new security concerns should be
introduced to Stateless DHCPv6 by the solution.
o It must be possible to update options, even if the network is not
renumbered.
o It is desirable to maintain the "stateless" property; i.e., no
per-client state should need to be kept in the server.
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RFC 4076 Renumbering for Stateless DHCPv6 May 20056. Solution Space
Solutions should be designed and presented in a separate document.
An initial brief set of candidate solutions might include the
following:
o Add a Reconfigure message mechanism that would work in the
stateless DHCPv6 environment. This could enable planned or
unplanned events, but may require a multicast mechanism in order
to be realised.
o Convey a valid lifetime timer to clients for stateless DHCPv6-
assigned settings. This could primarily enable planned events,
but with a small time-out it could handle unplanned events to some
extent at the expense of the additional request traffic. The
selection of recommended lifetime values/ranges would be the
subject of future work.
o Use some form of Router Advertisement (RA) [1] as a hint to
request new stateless DHCPv6-assigned settings. Using only an
observed new RA prefix as a hint to re-request settings would not
handle changes that are purely to NTP, DNS, or other options.
Other possible means of detection of network (re)attachment could
also be used as cues (e.g., see Goals of Detecting Network
Attachment (DNA) in IPv6 [6]).
o Change the semantics of the 'O' flag in RAs [2] so that toggling
its value may trigger an Information-request message.
There will also be conditions under which a client should send an
Information-request, such as reconnection to a link. Recommendations
for these cases are outside the scope of this document, but we expect
ongoing work in the DNA WG (as scoped in Goals of Detecting Network
Attachment (DNA) in IPv6 [6]) to yield recommendations.
7. Summary
This document presents a problem statement for how IPv6 hosts that
use the combination of Stateless Address Autoconfiguration and
stateless DHCPv6 may be informed of renumbering events or other
changes to the settings that they originally learned through
stateless DHCPv6. A short list of candidate solutions is presented,
which the authors hope will be expanded upon in subsequent documents.
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RFC 4076 Renumbering for Stateless DHCPv6 May 20058. Security Considerations
There are no security considerations in this problem statement per
se. However, whatever mechanism is designed or chosen to address
this problem should avoid introducing new security concerns for
(stateless) DHCPv6.
The issues of maintaining appropriate security through a renumbering
event are outside the scope of this document (if specific servers
within the network are being added or removed, firewall
configurations and ACLs, for example, will need to reflect this).
However, this is an important area for further work.
9. Acknowledgements
The authors would like to thank Ralph Droms, Bernie Volz, and other
individuals on the DHC mail list for their comments on this document,
as well as colleagues on the 6NET project. We also thank the review
comments, particularly those from Thomas Narten.
10. References10.1. Normative References
[1] Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery
for IP Version 6 (IPv6)", RFC 2461, December 1998.
[2] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[3] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M.
Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
RFC 3315, July 2003.
[4] Droms, R., "Stateless Dynamic Host Configuration Protocol (DHCP)
Service for IPv6", RFC 3736, April 2004.
10.2. Informative References
[5] Baker, F., Lear, E. and R. Droms, "Procedures for Renumbering an
IPv6 Network without a Flag Day", Work in Progress, July 2004.
[6] Choi, J., "Goals of Detecting Network Attachment (DNA) in IPv6",
Work in Progress, October 2004.
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RFC 4076 Renumbering for Stateless DHCPv6 May 2005
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